Surface contour information can be particularly useful for assessment of tooth condition and is helpful for various types of dental procedures, such as for restorative dentistry. A number of techniques have been developed for obtaining surface contour information from various types of objects in medical, industrial, and other applications. Optical 3-dimensional (3-D) measurement methods provide shape and depth information using light directed onto a surface in various ways. Among types of imaging methods used for contour imaging are those that generate a series of light patterns and use focus or triangulation to detect changes in surface shape over the illuminated area.
Fringe projection imaging uses patterned or structured light and triangulation to obtain surface contour information for structures of various types. In fringe projection imaging, a pattern of lines is projected toward the surface of an object from a given angle. The projected pattern from the surface is then viewed from another angle as a contour image, taking advantage of triangulation in order to analyze surface information based on the appearance of contour lines. Phase shifting, in which the projected pattern is incrementally shifted spatially for obtaining additional measurements at the new locations, is typically applied as part of fringe projection imaging, used in order to complete the contour mapping of the surface and to increase overall resolution in the contour image.
Color sensor arrays are more costly and complex than monochrome sensor arrays. In addition, sensor arrays that generate RGB data directly are inherently less efficient and less sensitive to low light level conditions, such as those common in intra-oral imaging.
In spite of cost, it can be appreciated that there would be value in providing 3-D surface contour images in color for both diagnostic and aesthetic purposes. Known approaches to this imaging problem provide ways to associate color to areas of the volume image. However, these approaches fall short of what is needed for providing color volume images that faithfully reproduce color texture. In general, texture of a surface relates to detailed surface structure and, in an imaging context, provides a more accurate representation of how light is reflected from the surface. Color texture also includes the spatial arrangement and intensity of color in the image. Attributes of color texture can be based on the directional distribution of reflected or transmitted light, typically described by attributes like glossy, shiny versus dull, matte, clear, turbid, distinct, or related to microsurface structure, such as roughness or smoothness, shading, and other attributes. Color texture representation is related to improved definition of edges, for example, and allows features within the mouth, and of the anatomy more generally, to be more clearly visualized.
Among proposed solutions for providing a measure of color information for 3-D images is that described, for example, in patent disclosure EP 0837659 entitled “Process and Device for Computer-Assisted Restoration of Teeth” to Franetzki, that obtains color data in a conventional manner using a color detector and then superimposes the 2-D Red (R), Green (G), and Blue (B) or RGB color image onto the 3-D volume image when it is displayed. This type of simulated color solution, however, does not provide true 3-D color image data. Provided that it can be correctly scaled and registered to the volume image data when overlaid onto the 3-D surface image, the simultaneously displayed and superimposed color content as described in EP 0837659, would be accurate at a single viewing angle only. Any other view of the 3-D surface would not have the superimposed color image content.
Thus, it can be appreciated that there is a need for an image processing method that provides 3-D image data of the teeth showing color as well as color texture content, using a single image capture apparatus that employs a monochrome sensor array.